Biosignatures in Earth and Mars ice and brines
Liquid water is essential to life on Earth...
... yet most planets and moons in our solar system have surface temperatures well below the freezing point of pure water. Earth itself may have undergone a series of global glaciations (Snowball Earth events) in its early history.
Mars is presently cold and dry, but orbital observations have identified flowing liquid water and likely past habitable environments. Its suggested that salty waters on Mars could host Earth-like life.
The abundance of icy conditions in our solar system suggests that life in frozen environments may provide answers to questions about the origin, evolution, and ultimate fate of microbial cells and their biosignatures. Bacteria that are growing, metabolically active, and/or surviving in the presence of salty ice and brines may have characteristics that reflect the evolution and nature of primitive life.
Devise three low-temperature liquid-water environments that mimic the known chemistry of brines: i) in sea ice on modern Earth, ii) on Snowball Earth, and iii) on Mars;
Measure microbial growth rate, metabolic activity, ability to survive while inactive, and longevity for psychrophiles in these three environments;
Reveal proteomic biosignatures for growth, activity, and survival strategies, and understand key molecular responses of life in these three environments.
Three strains of psychrophilic bacteria (Colwellia psychrerythraea strain 34H [Cp34H], Psychrobacter sp. strain 7E, and a model halophile) will be introduced to artificially-made brines and held for 12 months at -1, -5, -10, -20, -40, and -70°C.
The artificial brines will replicate the chemical composition of:
modern sea ice,
sea ice hypothesized to occur on Snowball Earth, and
ice on Mars (Preliminary work in our laboratories reveal that Cp34H is capable of growing in nutrient broth with Mars analog brine containing perchlorate).
To track metabolic activities we will spike cultures with 3H-Leucine, 3H-Thymidine, or 13C-Leucine and sample through time. Viability, cell death, and proteome changes will also be recorded and compared to optimal growth temperature (e.g. 8 °C for Cp34H). We expect that proteomic mass spectrometry (13C-Leucine ) will allow us to identify individual, newly synthesized proteins, which could serve as dominant biomarkers for growth and/or survival (Nunn et al., 2015).
This research has value to upcoming space-exploration life detection missions. Identification of proteins newly synthesized in low- temperature environments, or proteins indicative of long-term ice survival, will provide biosignatures to target when exploring life in low-temperature ecosystems relevant to future exploration. In addition, understanding key metabolic strategies for long-term survival in ice will provide clues on early evolution and survival of life as Earth underwent extensive glaciation during the Neoproterozoic Era.